I would ask whoever prescribed the Celexa that question if I were you.

no..u cant take any medicines or dieting pills...u need a good food whiich is nutritious..
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its because u cant able to maintain it...maintaining is very important ,,and by maintaining ur health u hav to knw abt nutrition ,,nutrition means protien,vitamins,minerals,fiber etc..if a person is of 1kg he need 1gm of protien daily,42 types of vitamins and minerals daily,,35 grams of fiber daily,,but we dont knw tht which food hav all this balanced nutrition ,,thts why we cant able to maintain weight,,we intake more and more calorie frm our normal indian food,,if a calorie is more thn 3 hrs in our body its converted into saturated and un saturated fats,if u need a balanced nutrition food thn for more info mail me on cooooolguy_212000@yahoo.co.in or call me on 09892256517..mumbai

You should be fine as long as the diet pill doesnt contain any stimulants like caffeine or Ephedra. But as the person above said check with your Doc.
Once they say ok, I suggest an all natural pill, have a look at http://www.thefatbinder.com
Good luck